The need often arises in antenna systems for conveniently altering the directionality of the system. It has long been conventional in the art to provide various types of mechanical steering systems whereby the boresight orientation of the antenna may be altered as in the familiar case of single parabolic dishes mechanically steered by a servomechanism.
Due to obvious disadvantages of these systems, such as their inherent mechanical complexity, other methods were sought for effecting beam steering. One approach involved a conventional phased array technique. In these systems, a plurality of phase shifting elements are arranged in an array, each element of which introduces a predetermined phase shift in the RF signal, thereby effecting steering of the beam as desired.
Although the problems of mechanical beam pointing are thus avoided, other problems are associated with phased array techniques. The requisite RF components are typically complex, expensive, and frequency dependent, thereby rendering reciprocal coincident operation at differing receive and transmit frequencies impossible. Moreover, characteristics of the phase shift elements themselves are problematical. Ferrite phase shifters, for example, are quite lossy with respect to switched time delay lines, prohibitive in physical size for some applications wherein they are arranged in serial fashion, and exhibit undesirable bandwidth and frequency limitations.
In yet another approach, to avoid some of the aforementioned problems, semiconductor control elements are employed for switching in various incremental time delay elements to steer a beam in a frequency independent manner. However, several additional problems are associated with this approach. First, desirable reciprocity in the transmit-receive modes is limited in that high powered transmit signals may either destroy the diodes or self bias them out of their switching mode, thus limiting the usable transmit power level. Moreover, in space communications applications and other applications wherein power consumption is important, switching of such diodes to provide multiple variable time delay increments is undesirable inasmuch as continuous power is required to operate them. Still further, such techniques have been limited to steering of single beams.
Thus, an antenna system was desired for multiple beam steering which was frequency independent relative to other systems, simple in construction, and capable of high power switching while at the same time exhibiting low operational power consumption requirements. Such a system was further desired which permitted reciprocal coincident operation for transmit-receive signal paths wherein the desired beam positioning could be easily specified, and was compatible with binary control signals.
Still further, such a system was sought which provided wide scanning coverage at moderate gain as well as a low gain mode, and favorable receiver-transmitter isolation as well as providing for aforementioned simultaneous coincident use for both forward and return links.
The disadvantages of the prior art hereinbefore noted are overcome by the present invention which will be described with reference to the accompanying drawings.